[RFI] Magnetic Field Interference

Brendan Minish ei6iz.brendan at gmail.com
Sun Mar 14 05:44:57 PDT 2010

Jim Brown wrote:

Hi Jim

thanks for the helpful reply

I have reduced the problem to the point where it's no longer an issue by 
moving the amp to the left of the operating position. Once I discoverd 
the issue I was keen to reduce the strength of the magnetic field in the 
vicinity of the K3 and it's transformer balanced line inputs.

 From a shack layout point of view it's not quite as good ergonomically 
but I can live with it

A bit of sniffing around with a solenoid coil and a scope helped me 
identify the best position for the amp. It's an interesting problem and 
I wonder how many of the issues one hears on air with Hum and other 
background noises is magnetically coupled

The magnetic field has quite a high harmonic content, Due I suspect to 
the non-linear nature of the load current because of the rectifiers in 
the HT side

> I know those headsets well -- for many years, I've specified them for 
> production intercom systems in the theaters for which I design sound 
> systems. 

This one has 50 ohm impedance ear pieces and is superb for CW (my 
primary mode) In the past I have spent many hours with DT-110's on 
mixing audio.

> Something else you can (and should) do with the K3 is roll off the low 
> octave bands in TXEQ. For most mics and most voices, the two lowest bands 
> should be full cut on the K3. That's 16dB for each band, and if they're 
> minimum phase filters, they'll combine to add a bit more. There's no useful 
> speech content in those two bands, but that's the spectrum where breath 
> pops and handling noise are prevalent. Indeed, many mics should also have 
> the next higher band cut as well. 

I fully agree, my Sole goal hare is for good quality communications 
audio that is not unpleasant to listen to.
The K3 EQ settings that I use by default are
50Hz -16
100Hz -16
200Hz -8
1.6KHz +3

I get good reports with this and these settings worked well with my 
other headset (DT-234)

> Another headset that works VERY well with the K3 (and most other ham rigs) 
> is the Yamaha CM500, which sells in the US for under $60 (I've seen it 
> discounted at $40). If that is available to you, it's worth considering. 
> Although some catalogs (including Yamaha's) list it as a dynamic, it's 
> actually an electret. 

The beyer DT 234 is a good choice that is affordable in Europe too but 
not as comfortable to wear as the DT-109 and the isolation is not as 
good. The rejection of shack noise (& my kids!) is a bit better with the 
DT-109 too

Brendan EI6IZ

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